Activation of oxalate metal coating compositions



Patented Nov. 11, 1952 ACTIVATION F OXALATE METAL COATING COMPOSITIONS Robert 0. Gibson, Birmingham, Mich assignor to Parker Rust Proof Company, Detroit, Mich., a; corporation of Michigan l Io Drawing. Application September'lS, 1951,

Serial No. 247,193

' This invention relates to a coating for the protection of stainless steel during mechanical working operations which result in extensive plastic deformation of the metal and more particularly relates to a method of activating the oxalatemetal coating compositions of the art for use on stainless steel during such deformation.

This is a continuation in part of the prior application, Serial 76,181, filed February 12, 1949, now .U. S. Patent 2,577,887, issued December 11, 1951, for improvements in Activation of Oxalate Metal Coating Compositions which is itself a continuation in part of the now abandoned application for an Oxalate Coated Article of Corrosion Resistant Metal and Method and Chemicals for Making Same, Serial 782,966, filed October 29, 1947.

- gested as so-called drawing compositions.

In this latter connection, it has been discovered that where the metal is to be drawn or otherwise subjected to deformation, an oxalate coating on the surface aids materially in protecting the surface against injury during such drawing or other deformation. 7

Not all metals or metal alloys have been found to be susceptible to the action of the suggested oxalate compositions of the art. As a eneral rule, metals which resist corrosion will resist the action of oxalate coating compositions. For example, nickel, chromium and alloys containing as much as 8% of either or both of these two metals are quite resistant to chemical action and, although a mixture of oxalic acid and ferric oxalate wil cause a coating to form thereon, the

coating is relatively non-adherent and dusty as compared with the coatings on iron.

It has already been taught that various ingredients may be added to an oxalic acid bath to speed up the rate of the reaction of the bath 1 on the metal such as, for example, ferric oxalate or the ferric ion. These added ingredients, some of which are otherwise known as oxidizing agents have been generally called accelerators. However, it is to be recognized that too rapid an attack on the metal may in itself be objectionable. A too rapid attack maycause an active pickling effect andthe production of no coating at all. It is; thereforefl-a considerable problem to increase the activity of a bath against the most resistantsta inless steel without destroyingthe 6 Claims. (01. 148-624) r 2 property of'the same-bath to coat effectively the less resistant metals.

For this reason, it is preferred to denominate the new bath ingredient of this invention an activator in view of the fact that it increases the activity of the bath on the most resist-ant stainless steels and at the same time has no adverse effect on the results secured with the more reactive metals. I 7

It is therefore an object of this invention to provide a method for the activation of Oxalate coating'compositions in-order that the latter will attack and effectively coat any of the stainless steels to produce a coating which will protect the surface of the metal during deformation operations.

It is a further object to provide a metal composition having incorporated therein a new agent capable of carrying out such activation.

Other objects and advantageous features of the invention will be specifically pointed out or will become apparent as the description proceeds.

In the said prior application, Serial 76,181, there is disclosed and claimed, a method of subjecting a stainless steel to deformation, which includes the steps of contacting the surface of the stainless steel with an acidic aqueous solution consisting essentially of oxalic acid, the ferric ion and an anion selected from the group consisting of chloride and bromide until a substantial coating forms on the surface of the stainless steel, the chloride ion or its equivalent being present in an amount ofat least about 2% and thereafter subjecting the stainless steel to deformation. Additionally, the same application discloses and claims the composition which is used in carrying out the above method.

The present invention is concerned generally with an oxalic acid solution containing the ferric ion and is directed to still another activating anion which has-been found to satisfactory activate such solutions and produce a coating on the most resistant stainless steels. This invention furthermore concerns the method of subjecting a stainless steel surface to the action of the activated solution of this invention and thereafter deforming the said stainless steelsurf-ace. Thus, this invention contemplates a composition for coating a stainless steel to protect the surface during deformation operations such as drawing and the like which comprises an acidic aqueous oxalate metal coating composition having incorporated therein the thiocyanate ion in a proporproportion of ferric oxalate increases the solu-' bility therein of ferrous oxalate increases materially.

The ferrous oxalate may be put into solution by an agent which leaves no' objectionable residue and which furthermore has no effect upon the coating characteristics of the solution. It is preferred, although not necessary, that the amount of oxidizing agent used be less than that necessary to oxidize all of the ferrous ion to ferric ion. Hydrogen peroxide is an example of such an oxidizing agent which has been utilized successfully. When ferrous oxalate partly saturates the ferric oxalate solution, the coating deposits more readily, but at greater concentrations of ferrous oxalate a point is reached at which the deposited coating tends to be loose and not satis factorily adherent.

When the metal surface'is first subjected to the action of the bath, the deposition of coating is relatively rapid. However, a point is reached where there is maximum coating and-after that further processing may reduce the'coating thickness. In other words, the rateof etching ofthe metal and to some extent the rate of redissolving of'the ferrous oxalate inthe bath increases in proportion to the rate of deposition of coating as time goes on.

The preferred solution is one which comprises about .4 to iron and 1.5% to 40% thiocyanate ion in an aqueous'oxalic acid solution. It is'preferred that the amount of oxalate ion be in excess of that which is required toform'ferri'c oxalate. It is also preferred that the'iron and the oxalate be derived from a mixture of oxalic acid and ferric oxalate or a mixture offerric oxalate and' a'small percentage of an acid other than" oxalic acid. It will beapparentin thelatter case that ionization-of the ferric oxalate will result in the presence of oxalic'acid in the solution an'dit'is' for this reason that'ferricoxalatemayserve as-a source of the'coating ingredients or may serve as'a source of accelerating ion -inthe oxalic acid bath. 7

A typical coating solution inaccordance'with this invention may be compounded by placing-9.6 gramsof ferrousoxalate and 19.0 gramsoxalic acid in water to make 200 ml. The ferrous oxalate may then be oxidized to ferric oxalate by slowly adding hydrogen peroxide and observing the disappearance of ferrous oxalate precipitate.

To this solution, varying proportions of potassium thiocyanate were added tomake separate solutions in which stainless steel panels could be immersed to determine the coating formation characteristicsof thevarious solutions. Stainless steel panels having a compositionof substantially 18% chrome, 8% nickel and 74% iron-were immersed in the above'basicsolution which had been modified byadding thereto potassium thiocyanate in amounts sufficient" to produce concentrations of 5%, 0%, 13%, 20% and 40%.

The use of these various solutions for the processing of large quantities of work somewhat depletes the acidity ofthe-Solutions and oxalic acid should be added from time-to time to main 4 tain the original acidity. Similarly, additional quantities of hydrogen peroxide should be added periodically to prevent the formation of ferrous oxalate precipitate and/ or to return any precipitated ferrous oxalate to the solution as ferric oxalate.

All of the solutions enumerated above produced satisfactory adherent yellowish green to light gray coatings within the range of temperatures from room temperature to the boiling point of the'solutions. A wide variety of stainless steel available for test purposes was immersed in these solutions and coatings formed thereon. The

term stainless steel is used here in its generally accepted meaning. A detailed list of steels which come under the term stainless steel can be found on pages 554 and 555 of the 1948 edition of the Metals Handbook published by the Amerioan society for Metals.

The above coatings were formed on the panels in each of the above identified solutions in a period of time from about 3 to 15 minutes with satisfactory coatings being produced in most instances in 3 to 10 minutes. Increases in the concentration of iron required an increase in the quantity of thiocyanate above the minimum percentage of 1.5%, for example, at'an iron concentration of .4%, 1.5% thiocyanate is sumcient; at an iron concentration of 1.5%, 2% of thiocyanate is required; at an iron concentration of 6%., the thiocyanate required was 20%, and at an iron concentration of 15%, 40% of thiocyanate was required to forma'coating. As mentioned above, the quality of the coating formed is not enhanced by the presence of an excess of thiocyanate inasmuch as the threshold quantity of thiocyanate is extremely critical and a coating, if obtainable at all, is of excellent quality. High percentages 'of thiocyanate are somewhat undesirable from a practical operating standpoint and it is preferred that'the ferric iron concentration be maintained between .4% and 6% so' that the required percentage of thiocyanate will not exceed about 20%.

The dry chemicals as'listed above excluding chlorates and/or bromates may be mixed in about the proportions" given aid'used'to make up a coating solution, or the chemicalsmay be first dissolved and reacted ina limited amount of water and then diluted for use. Since some ferrous oxalate formed in'the coating operation fails to adhere firmly to the metal'being coated and tends to saturate the solution and form sludge, such ferrous oxalate'may'be reoxidized and therefore relatively reduced amounts offerrous oxalate are required 'inreplenishing. Therefore, whether introduced as a concentrated solution, as mixed dry chemicals, or as separate chemicals, replenishment is ordi'narily by'oxalic acid, thiocyanate; and ferrous oxalate. Hydrogen peroxide may be added in an amount to oxidize the desired quantity of-fer'rous oxalate to ferric oxalate; The'thiocyan'ate ion maybe supplied in any form except that it is preferred to avoid the addition of certain cations-with the thiocyanate where there is an incompatibility of the cation with the bath ingredients. Compatiblesaltswhichmay organic dope prior to the drawing. Equally good results can be obtained with oxalate coatings in less time-consuming treatments at much less cost and with the elimination of many difficulties incident to the dope or lead processes. The new method requires merely the formation of the oxalate coating, after which the metal may be worked with the application of suitable drawing compounds such as soap. After drawing, the coating can be easily removed by immersing the work for a short time in an acid pickle. However, with some lubricants it has been found desirable to immerse the coating in an alkaline cleaner prior to the acid pickle.

All percentage figures in this specification and in the accompanying claims are percent w./v.; that is, the solution is assumed in each instance to have a specific gravity of unity.

What is claimed is:

1. A composition of matter for coating a stainless steel to protect the metal during a deformation operation such as drawing and the like consisting essentially of an acidic aqueous solution of oxalic acid, the ferric ion and the thiocyanate ion, said ferric ion being present in an amount of about .4 to and said thiocyanate ion being present in an amount of 1 /2% to l0%.

2. A composition of matter for coating a stainless steel to protect the metal surface during a deformation operation such as drawing and the like which consists essentially of an acidic aqueous solution of oxalic acid, the ferric ion and the thiocyanate ion, the oxalate ion being present in an amount in excess of that required to form ferric oxalate, the ferric ion being present in an amount between about .4% and 15% and the thiocyanate ion being present in an amount between about 1.5% to about 40%.

3. A composition of matter for coating a stainless steel to protect the metal surface during a deformation operation such as drawing and the like which consists essentially of an acidic aqueous solution of oxalic acid, the ferric ion and the thiocyanate ion, the oxalate ion being present in an amount in excess of that required to form ferric oxalate, the ferric ion being present in an amount between about .4% and 6% and the thiocyanate ion being present in an amount between about 1.5% and about 20%.

4. In a method of subjecting a stainless steel to deformation such as by drawing and the like, the steps of subjecting the surface of the stainless steel until a substantial coating forms to the action of an acidic aqueous solution consisting essentially of oxalic acid, the ferric ion, and the thiocyanate ion, the oxalate ion being present in an amount in excess of that required to form ferric oxalate, the ferric ion being present in an amount between about .4% and 15%, the thiocyanate ion being present in an amount between about 1 and 40% and thereafter subjecting the stainless steel to said deformation.

5. In a method of subjecting a stainless steel to deformation such as by drawing and the like, the steps of subjecting the surface of the stainless steel until a substantial coating forms to the action of an acidic aqueous solution consisting essentially of oxalic acid, the ferric ion, and the thiocyanate ion, the ferric ion being present in an amount of about .4 to 15% and the said thiocyanate ion being present in an amount of 1 /2% to 40%, and thereafter subjecting the stainless steel to said deformation.

6. In a method of subjecting a stainless steel to deformation such as by drawing and the like, the steps of subjecting the surface of the stainless steel until a substantial coating forms to the action of an acidic aqueous solution consisting essentially of oxalic acid, the ferric ion, and the thiocyanate ion, the oxalate ion being present in an amount in excess of that required to form ferric oxalate, the ferric ion being present in an amount between about .4% and 6%, the thiocyanate ion being present in an amount between about 1 and 20% and thereafter subjecting the stainless steel to said deformation.

ROBERT C. GIBSON.

No references cited. 

1. A COMPOSITION OF MATTER FOR COATING A STAINLESS STEEL TO PROTECT THE METAL DURING A DEFORMATION OPERATION SUCH AS DRAWING AND THE LIKE CONSISTING ESSENTIALLY OF AN ACIDIC AQUEOUS SOLUTION OF OXALIC ACID, THE FERRIC ION AND THE THIOCYANATE ION, SAID FERRIC ION BEING PRESENT IN AN AMOUNT OF ABOUT 4 TO 15% AND SAID THIOCYANATE ION BEING PRESENT IN AN AMOUNT OF 1 1/2% TO 40%. 